A CPU clock frequency and an operation clock for CPU interaction with external devices recently has been remarkably increased so that a processing speed of a computer may be increased. With the speed increase of the operation clock, the performance requirement for the printed wiring board where a data signal runs also become demanding. By way of example, there is a requirement that the characteristic impedance between printed wiring circuits formed by signal conductors on the printed wiring board need to be matched and that the characteristic impedance and the impedance of the circuit load need to be matched. Without the match, a signal is partially reflected back to a signal source such that the signal transmitted to the load becomes so weak that the data may not be transmitted completely. This becomes more important with an increase of the frequency.
Thus, to maintain the quality of the printed wiring board, it will be important to measure the characteristic impedance of the printed wiring circuit. Conventionally, the characteristic impedance of the printed wiring circuit is measured manually using a coaxial probe.
The operation clocks of equipment connected to computers are expected to increase in speed further in the future. Along with this trend, the need to measure the characteristic impedance to assure the quality of the printed wiring board will increase significantly. Then, the conventional method of manually measuring the impedance of the printed wiring circuit using a coaxial probe may not be good enough. It will be necessary to automate this process.
Thus, it is efficient and desirable to move a probe unit by means of a testing device for the printed wiring board to measure impedance between testing points. Therefore, an object of the present invention is to provide an impedance measuring device for the automatic measurement of the characteristic impedance of the printed wiring circuit on the printed wiring board principally by loading a coaxial probe, or another type of the probe in the automatic printed wiring board testing device.